The present invention relates generally to an electrochemical device, and more particularly to a device which comprises a laminated electrochemical cell having planar solid electrolyte bodies.
There have been known various electrochemical devices using a solid electrolyte, for example as oxygen sensors to detect the oxygen concentration of an exhaust gas from internal combustion engines of automotive vehicles. Typical examples of such oxygen sensors include an oxygen sensor which comprises a tubular body of an oxygen-ion conductive solid electrolyte such as zirconia ceramics, and electrodes of platinum or the like provided on both inner and outer surfaces of the tubular solid electrolyte body, and which operate to determine the oxygen concentration according to the principle of an oxygen concentration cell. Another example is an oxygen sensor similar to the above, which incorporates a heater enabling the sensor to operate at a relatively low exhaust gas temperature. A so-called "lean-burn" is suitable for detecting the oxygen concentration of an exhaust gas which is produced at lean air-fuel ratios, i.e., an exhaust gas having a larger oxygen content. Also known in the art are electrochemical devices such as sensing and pumping elements for detecting hydrogen, nitrogen, carbon dioxide gas, etc. These sensing and pumping elements operate according to the principle of a concentration cell, like the oxygen sensor indicated above.
In such electrochemical devices, solid electrolyte materials have been generally used in the form of a tubular body which has an elongate bore closed at its one end. In recent years, however, it has been attempted to replace the tubular solid electrolyte body with a solid electrolyte body of planar shape, as disclosed in U.S. Pat. No. 4,334,974, in view of the relatively low productivity and high cost of manufacture of the solid electrolyte bodies of tubular shape, and from the standpoint of easy assembling of parts with the solid electrolyte body. When such planar solid electrolyte bodies are employed, suitable electrodes are disposed in contact with the surfaces of the planar body of the solid electrolyte, and the electrolyte bodies and other parts are assembled into a laminar structure constituting an electrochemical cell.
In such an electrochemical cell, wherein planar or plate-like bodies of a solid electrolyte are laminated, each of the conductors such as a heating element of a heater and leads for the electrodes and the heating element is sandwiched between a pair of adjacent planar solid electrolyte bodies (i.e., the conductors are embedded in the mass of the solid electrolyte). Accordingly, the conductors are held in direct contact with the solid electrolyte. Since the electrochemical device is adapted such that a voltage is applied, for example, to an oxygen pumping element through such conductors (conductive leads), parts of the solid electrolyte contacting the conductors are electrolyzed and tend to deteriorate. Thus, the solid electrolyte bodies have potential problems of cracking and deterioration or loss of the ion conductivity and other characteristics. Further, the electrolysis of the solid electrolyte may cause an erroneous measurement between the conductor and a measuring electrode, which affects an electromotive force to be measured, and causes a measurement error of the electrochemical device. In addition, an air gap exists between the conductor and the solid electrolyte bodies sandwiching the conductor. This air gap accommodates an oxygen gas, which may cause degradation of the response and generation of output noises of the sensing device. In the case where a heater is embedded in the solid electrolyte for heating exhaust or other gases to be measured when their temperature is low, the heater tends to serve as a conductor, and therefore, may cause problems as indicated above.